A numerical analysis of biogeochemical controls with physical modulation on hypoxia during summer in the Pearl River estuary

Abstract

Abstract.A three-dimensional (3-D) physicalbiogeochemical coupled model was applied to explore the mechanisms controlling the dissolved oxygen (DO) dynamics and bottom hypoxia during summer in the Pearl River estuary (PRE). By using the numerical oxygen tracers, we proposed a new method (namely the physical modulation method) to quantify the contributions of boundary conditions and each source and sink process occurring in local and adjacent waters to the DO conditions. A mass balance analysis of DO based on the physical modulation method indicated that the DO conditions at the bottom layer were mainly controlled by the source and sink processes, among which the sediment oxygen demand (SOD) at the water-sediment interface and the re-aeration at the air-sea interface were the two primary processes determining the spatial extent and duration of bottom hypoxia in the PRE. The SOD could cause a significant decrease in the bottom DO concentrations (averaged over July-August 2006) by over 4 mg L −1 on the shelf off the Modaomen sub-estuary, leading to the formation of a high-frequency zone of hypoxia (HFZ). However, the hypoxia that occurred in the HFZ was intermittent and distributed in a small area due to the combined effects of re-aeration and photosynthesis, which behaved as sources for DO and offset a portion of the DO consumed by SOD. The bottom DO concentrations to the west of the lower Lingdingyang Bay (i.e. the western shoal near Qi'ao Island) were also largely affected by high SOD, but there was no hypoxia occurring there because of the influence of re-aeration. Specifically, re-aeration could lead to an increase in the bottom DO concentrations by ∼ 4.8 mg L −1 to the west of the lower Lingdingyang Bay. The re-aeration led to a strong vertical DO gradient between the surface and the lower layers. As a result, the majority (∼ 89 %) of DO supplemented by re-aeration was transported to the lower layers through vertical diffusion and ∼ 28 % reached the bottom eventually. Additional numerical experiments showed that turning off re-aeration could lead to an expansion of the hypoxic area from 237 to 2203 km 2 and result in persistent hypoxia (hypoxic frequency > 80 %) to the west of the lower Lingdingyang Bay. Compared to re-aeration and SOD, photosynthesis and water column respiration had relatively small impacts on the DO conditions; turning off these two processes increased the hypoxic area to 591 km 2 . In summary, our study explicitly elucidated the interactive impacts of physical and biogeochemical processes on the DO dynamics in the PRE, which is critical to understanding hypoxia in this shallow and river-dominated estuarine system.

Carbon System Simulation in the Pearl River Estuary, China: Mass Fluxes and Transformations

“…The total depositional flux of POC (including phytoplankton and nonliving POC) into sediment is 7.00 × 10 10 mol C year −1 , which is higher than the POC input from the river (5.59 × 10 10 mol C year −1 ). The deposition also results in high sediment oxygen demand in summer (Wang et al, ) and increases the DIC flux from the pore water of sediment to the overlying water in the PRE as mentioned earlier. The death of phytoplankton contributes 2.33 × 10 10 mol C year −1 of POC annually, which supports a relatively smaller part of the deposition of nonliving POC (with a maximum of 35.7%) compared to the deposition of terrestrial POC.…”

Physical dynamics structures and oxygen budget of summer hypoxia in the Pearl River Estuary

“…). The total bacterial respiration in the bottom mixed layer is estimated to be h BML R C = 7.38 g m −2 d −1 with h BML = 11.9 m. The value of SOD at the mooring site C is ~ 0.6 g m −2 d −1 (Wang et al ). In a word, all DO losses in the bottom mixed layer are 9.03 g m −2 d −1 , whereas the DO supply only by the horizontal advection from shelf benthic waters is 9.89 g m −2 d −1 .…”

“…Moreover, the coastal hypoxia is forecast to increase owing to the combined effects of continued spread of coastal eutrophication and global warming largely associated with human activities (Diaz and Rosenberg ; Vaquer‐Sunyer and Duarte ). More than 400 coastal systems of hypoxia (termed “dead zone”) have been identified, with a total area of more than 245,000 km 2 (Diaz and Rosenberg ), e.g., Gulf of Mexico (Rabalais et al ; Zhang et al ), Chesapeake Bay (Hagy et al ; Murphy et al ), Lower St. Lawrence Estuary (Bourgault et al ; Lefort et al ), Yangtze River (Changjiang) Estuary (Chen et al ; Wei et al ), and Pearl River (Zhujiang) Estuary (Yin et al ; Luo et al ; Wang et al ). While it is generally accepted that this increase was the result of increased anthropogenic nutrient loads to the estuaries or coastal ocean, studies that attempt to statistically relate the inter‐annual variations in hypoxic volume to nutrient loading often fail to explain the majority of the variability (Hagy et al ; Murphy et al ; Scully , ).…”

“…Since summertime hypoxia in the Pearl River Estuary (PRE) was first reported in 1970s to 1980s, it has attracted a widespread attention of ocean scientists and public administrators (Yin et al ; Luo et al ; Zhang and Li ; Wei et al ; Wang et al ). The seasonal hypoxia occurred on the west and east shoals in the Lingdingyang Estuary, and in the Modaomen Estuary (Cai et al ).…”

Hypoxia in deep waters of moderately eutrophic marine lakes, Island of Mljet, eastern Adriatic Sea

“…That would be an explanation for the negative correlation (p<0.05) between NH 4 and O 2 in SL (Table 3). Wang et al (2017) reported that re-aeration and sediment oxygen demand were more important than photosynthesis and water column respiration as counterbalanced coupled processes affecting the near-bottom DO concentration in the shallow estuary. Re-aeration of deep waters in the MLs is not expected in summer (Vilibić et al 2010).…”